The literature describes many ways of preparing gabapentin from a variety of starting materials, but there is no suggestion of the preparation of gabapentin tannate. U.S. Pat. No. 4,024,175 describes at least three methods of preparing gabapentin from cyclohexyl-1, 1-diacetic acid. Each of these methods results in the formation of gabapentin hydrochloride salt, which may be converted to I-(aminomethyl)-I-cyclohexaneacetic acid by treatment with a basic ion exchanger and then crystallized from a solvent such as ethanol/ether.
The prior art neither discloses nor suggests the preparation of gabapentin tannate. The formation of a tannate salt of gabapentin is unexpected because of the close proximity of a carboxylic acid group to the amine group. The negative charge on the carboxylic acid group was expected to shield and possibly neutralize the positive charge on the proximal nitrogen. Since tannate salts are thought to normally form through an ionic interaction with a positively charged amine functional group, the close proximity of the carboxylic acid group was expected to prevent the formation of the tannate salt.